Recent increased concern associated with environmental problems and energy depletion has brought about an increased interest in solar cells as alternative energies which are abundant in energy sources, are free of environmental contamination and provide high energy efficiency.
Solar cells are classified into solar heat cells to generate vapor required for rotation of turbines using solar heat and solar light cells which convert sunlight (photons) into electric energy using semiconductor characteristics. Of these, solar light cells which convert light energy into electric energy absorb light to produce electrons and holes are actively researched.
Such a solar light cell (hereinafter, simply referred to a “solar cell”) is schematically illustrated in FIG. 1. Referring to FIG. 1, the solar cell includes a first conductive semiconductor layer 22, a second conductive semiconductor layer 23 having an opposite conductive type to the first conductive semiconductor layer 22 arranged thereon, a P/N junction formed between the first and second conductive semiconductors, a rear electrode 21 which contacts at least a part of the first conductive semiconductor layer 22, and a front electrode 11 which contacts at least a part of the second conductive semiconductor layer 23. In some cases, the solar cell may further include an anti-reflection film 24 arranged on the second conductive semiconductor layer 23.
The first conductive semiconductor layer 22 is generally a p-type silicon substrate and the second conductive semiconductor layer 23 is generally an n-type emitter layer. In addition, the front electrode 11 is formed with an Ag pattern on the emitter layer 23, and the rear electrode 21 is formed with an Al layer on the rear surface of the silicon substrate 22. The formation of the front electrode 11 and the rear electrode 21 is generally carried out by a screen printing method. The front electrode is generally composed of two current-collecting electrodes having a large width (also called a “bus bar”) and a grid electrode (also called a “finger”) having a small width of about 150 μm.
In such a solar cell having this configuration, when sunlight is incident upon the front electrode 11, free electrons are generated and move toward the n-type semiconductor layer 23 based on the principle of the PN junction and this flow of electrons forms a current.
As such, performance of solar cells which directly convert light energy into electric energy is represented by a ratio of electric energy emitted from the solar cells to incident solar energy. This ratio is an indicator of performance of solar cells and is commonly referred to as “energy conversion efficiency”, or simply “conversion efficiency”. The theoretical limit of conversion efficiency depends on constituent components of solar cells and is controlled by the spectrum of sunlight and sensitivity spectrum of the solar cell. For example, monocrystalline silicon solar cells have a conversion efficiency of about 30 to 35%, amorphous silicon solar cells have a conversion efficiency of 25% and compound semiconductors have a conversion efficiency of 20 to 40%. However, actual efficiency of solar cells is currently about 25% on a laboratory scale.
The reasons behind this may be loss of surface-reflective light, loss of carriers by recombination on the surface or interface of electrodes, loss of carriers through recombination in photocells and loss by internal resistance of solar cells.
Of these reasons, power loss by electrodes include resistance loss caused by movement of photocurrent on the n-type semiconductor layer, loss by contact resistance between the n-type semiconductor layer and the grid electrode, resistance loss by photocurrent which moves along the grid electrode, and loss by regions shielded by the grid electrode.
However, some factors associated with power loss are contrary to one another. For example, resistance loss is inversely proportional to the thickness of a grid electrode, but loss of the amount of incident light (the amount of absorbed light) is directly proportional to the size of the grid. Accordingly, when the size of the grid electrode is increased in order to minimize resistance loss, loss of incident light disadvantageously increases.
Accordingly, there is an increasing need for methods which minimize power loss by electrodes and maximize light absorbance amount in order to realize high-efficiency solar cells.